Not true at all. Ive run spacers on cars for years. If they're installed properly, you should have ZERO vibrations at any speeds. I run spacers right now on my winter setup and have hit these high speeds in question without any vibrations. Your friends are just dead wrong.
2016 Individual M5
2015 F86 X6M
2015 i3-REX- 2012 Wrangler Unlimited